Electron microscopic examination of two chordomas provided evidence to clarify the histogenesis of the cytoplasmic vacuoles which characterize the light microscopic appearance of some of their cellular elements. Small vacuoles found in cells with “foamy cytoplasm” appear to represent dilated cisterns of coarse endoplasmic reticulum, whereas large forms observed in the so-called physalipherous cells are derived from herniations or sequestrations of interstitial material. Entrapment of portions of cytoplasm from cells might also artifactually appear as vacuoles by light microscopy. Transitions between the fusiform, nonvacuolated, and the round, overtly-vacuolated cellular elements of chordomas indicate that these cells are histologic variants of the same cell type, as well as showing the progenitor nature of the former. Recognition of similarly-structured vacuoles, desmosomal attachments, cytoplasmic filaments, and abundant pinocytic vesicles in samples of rat notochord represent morphologic evidence supporting the view relating chordomas to notochordal elements, and also indicate their epithelial (ectodermal) nature.